The present invention relates to apparatus for processing flowable plastic materials, and more particularly to improvements in apparatus for removably supporting one or more filters between one or more extruders and one or more molds for plasticized synthetic plastic material. Still more particularly, the invention relates to improvements in filter changing apparatus for plastic materials and to improvements in methods of manipulating or operating such filter changing apparatus.
U.S. Pat. No. 4,597,870 to Lambertus discloses a filter device for a screw extruder wherein a housing supports a reciprocable carrier or slide member for a composite tubular filter or sieve. The carrier can move the filter to and from an operative position in which heated plastic material can flow from an inlet to an outlet of the housing by passing radially inwardly through the interstices of and into the interior of the filter to leave at one axial end of the filter on its way toward an outlet of the housing. A drawback of the patented apparatus is that pressurized plastic material which flows from the inlet of the housing, radially inwardly into the filter and thereupon axially and out of the filter toward the outlet of the housing is likely to reduce the sizes of interstices for the flow of plastic material into the filter. Moreover, as the filter gathers a certain amount of impurities, its resistance to the flow of plastic material from the inlet of the housing toward the interior of the filter increases with attendant rise in pressure of conveyed plastic material, i.e., the rising pressure is even more likely to reduce the sizes of interstices in the filter. This, in turn, enables the intercepted impurities or contaminants to further reduce the combined cross-sectional area of unobstructed interstices so that the filter must be inspected, removed and cleaned, or discarded and replaced with a fresh filter, after a relatively short interval of use. Moreover, elevated pressure is often detrimental to the plastic material; for example, the chemical composition of certain thermoplastic materials is changed or the material is actually destroyed when the pressure upon such material reaches a predetermined value. Still another drawback of a filter which is installed in the path for the flow of plastic material in such a way that the plastic material must flow radially inwardly in order to penetrate into the interior of the filter is that the intercepted contaminants gather at the exterior of the filter. Such deposits of contaminants interfere with movability of the reciprocable carrier or slide member which is used to move the filter to and from its operative position. As a rule, the carrier is a cylinder which is a snug fit in a cylindrical bore or hole of the filtering apparatus.
U.S. Pat. No. 3,455,357 to Zink discloses a filter or screen changer apparatus wherein a pivotable or rockable pendulum type carrier supports two tubular filters one of which is accessible when the other is in actual use and the other way around. A drawback of pivotable or rockable carriers for filters which are used to intercept impurities in a stream of heated plastic material is that proper sealing of the carrier in the housing presents many problems and that the apparatus is rather complex and bulky.
German Auslegeschrift No. 1 097 660 of Corbett discloses an apparatus which is similar to that of Zink. Thus, a cylindrical filter is mounted for angular movement in a housing which is provided with an inlet for unfiltered plastic material and an outlet for filtered plastic material.
German Auslegeschrift No. 2 111 545 of Lambertus discloses an apparatus wherein a plate-like carrier supports several axially parallel filters. The carrier is movable between a plurality of positions in each of which a different set of filters can receive plasticized material from an extruder.
Commonly owned U.S. Pat. No. 4,725,215 to Kreyenborg et al. discloses an apparatus with an exchangeable cylindrical filter holder which is reciprocable in a housing and contains a substantially plate-like filter disposed substantially diametrically of the holder.
An apparatus with several filter holders of the type disclosed by Kreyenborg et al. is described and shown in commonly owned copending patent application Ser. No. 07/446,557 filed Dec. 5, 1989 for "Apparatus for filtering plasticized materials in extruders and like machines", now U.S. Pat. No. 5,004,414 granted Apr. 2, 1991.